In the following specification and claims, the expression frame, span, and bracket, including their derivatives in various forms, will be used interchangeably and are to be understood to refer to one and the same measure of target image extent and are to be understood to include any measurement parameter useful to the various applications of the instant invention.
This invention relates to optical devices for surveying, rangefinding and the like and is particularly adaptable for scopes and their use in aiming firearms and the like.
Since the advent of firearms, ways and means have been sought to accurately aim these devices. Evolving from simple notch-and-bead arrangements, the optical riflescope has proved to be the most effective device for the fulfillment of this task.
Riflescopes have been commercially available for many years and have remained largely unchanged since their introduction. These instruments comprises two basic types. The simplest form of riflescope constitutes a body tube or housing, objective and ocular lenses disposed at opposite ends thereof, and a pair of erector lens cells mounted at opposing ends of an erector lens tube situated in the space between the objective and ocular lenses and serving to erect the simple image formed by the objective lens. It is usual that the erector optics tube is controllably pivotable by adjustment screws for the positional fine adjustment of the target image relative to a pair of aiming crosshairs or reticle disposed at the rear plane of focus of the erector optics. The riflescope is fixedly attached to a firearm to provide an image of the object of the operator's interest.
In a second and more complex form of the riflescope, the erector optics are controllably movable along the optical axis to provide an image of varying magnification. A ring or knob external of the body tube is manipulated by the operator to alter, at will, the size of the image in the riflescope within the limits afforded by the design and structure of the particular scope. The range of a zooming riflescope (as it is commonly called) is typically 3.times. to 9.times. which means that at the lowest power setting the image is three times lifesize while at the highest setting it is nine times lifesize.
In addition to lens elements, a reticle or crosshair arrangement is provided to assist aiming. The body housing or tube is provided with mounting bracket means by which the riflescope is affixed to the firearm. Once mounted upon a firearm, the riflescope is "zeroed", an operation which establishes a precise relationship between the bore of the firearm and the operator's line of sight through the riflescope. This adjustment is facilitated by means of a horizontal (windage) adjustment and a vertical (elevation) adjustment. "Zeroing" is done in relation to an impact point at a known distance. For instance, a firearm may be "zeroed" so that the aiming reference, the reticle or crosshairs, is aligned with a point one hundred yards distant and is coincident with the placement or location of the projectile at that range. Once established, the alignment is permanent until such time as the rigors of field use necessitate the re-establishment of alignment.
In riflescopes of basic construction, differences in range of actual targets must be compensated for entirely by the operator who first estimates the target's range and then imposes a degree of holdover deemed to be compensatory of projectile drop at that distance. Few shooters become adept at estimating range and at compensating for bullet drop. In the field, variations in terrain, air quality, and ambient light can cause even the best of shooters to misjudge target range a significant percentage of the time resulting in their often being off the mark when firing. Additionally, shooters who are expert at such judgment are able to attain accuracy with only one or two specific cartridges and rifles with which they have become familiar by practice.
Limited static and dynamic means have evolved for range determination and aiming compensation based on the fact that a target's optical image in the riflescope's field of view diminishes in size hyperbolically in proportion to its distance from the operator.